app.less contains imports for all the Bootstrap 2.3.2 stylesheets, but since Bootstrap is in bower_components, Brunch also compiles that version into app.css. So app.css is twice as large as it needs to be, but also the app-specific CSS in _overrides.less is unable to override Bootstrap styles, because Bootstrap ends up both before and after it in the compiled CSS.
I resolved this in my repos by removing all the Bootstrap imports from app.less (making it really just for app-specific CSS), and ensuring that Brunch adds app.less after the vendored CSS in config.coffee: see tdumitrescu/angular-express-coffee-blog@12bffdb.